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Carrier-Sense Medium Access Reference Design (CSMAMAC)
The CSMAMAC is serves as the user-level MAC layer in reference design. The basic algorithm is simple:
- Do I have a packet to send?
- If the medium is idle, send it, enter a timeout, and wait for an acknowledgment from the destination
- If the medium is busy, enter a backoff period and wait for the medium to become idle
- Did I receive a data packet?
- If the packet pass checksum and is addressed to me, send an acknowledgment
- Did no know acknowledgment happen during a timeout period?
- If the maximum number of retransmits has not occurred, enter a backoff and try retransmitting
- If the maximum number of retransmits has occurred, drop the packet
- Did I wait through a backoff period?
- If the medium is busy, retransmit, increment the total number of resends, enter a timeout, and wait for an acknowledgment from the destination
- If the medium is busy, enter a backoff period and wait for the medium to become idle
This simple algorithm encompasses much of the behavior in commercial 802.11 MAC/PHY chipsets. This algorithm lends itself nicely to a state diagram, which in turn, translates into C-code that can be executed in the PowerPC of the FPGA on WARP. In the following section, we track the "Life of a Packet" as it might journey through these states.
Life of a Packet
Below is the state-machine representation of the above algorithm. Each state can be "clicked" and is described in detail in the accompanying link. The up-to-date code can be found here: csmaMac.c. The API documentation for the code can be found here: CSMAMAC API
Idle
There are two purposes of this state: At the beginning of time the code must initialize the framework and WARP hardware, and in the steady state it must sit and wait for an event to take place. This state maps to the
int main()
function of csmaMac.c.
